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Dreamcast [DEV] : MOD Dreamcast REV0 (Va0) with GDEMU

therobotx5

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Mar 14, 2021
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Hi, I recently bought a DC signature model from Japan. Would also like to do this mod. Are all Chinese Gdemu the same hardware?. Has anyone had any adverse affects from doing the mod? Much better than forking out £200 for a MODE. Cheers :)
I'm pretty sure they are all the same just with minor component variations such as the the sd card slot. My GDEMU is labeled "GDEMU v5.15b 2020-08-24" with the closed SD card slot I got on Ali express. I have attached an image of what I got. It is not yet tested since I am waiting for the 100 RAs. In terms of any adverse effects, what I have seen people do is add a 300 ohm 1 watt resistor to the 12 volt line and ground or remove the 12 volt regulator entirely. This is because there are no longer any components using the 12 volt line of the psu and all it it does is generate more heat. But this is not required it is just to reduce heat. I decided to remove the regulator entirely if you want to know what I did.PXL_20210324_195737767.jpg
 

deadlegion

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With my Japanese VA1 I pulled the original PSU and dropped in a pico that I got for around $25aud, already had an external 12v power brick (around 4A, but 2A does work) suitable for the pico so this is pretty much all I spent.
Clean the power connector pins but you shouldn't really use sandpaper to do that, you can use deoxit but I just used isopropyl alcohol.

If you've pulled your DC apart it's a good time to replace the controller board battery with a holder, don't use CR2032 batteries when you replace though make sure you use rechargeable 3V type like ML2032. You can replace the controller board fuse too (with a reset type poly switch) if you like but I haven't bothered atm, mine hasn't blown at any point from what I can tell anyway.

Another thing to do is general cleaning and maybe replace the thermal pads top and bottom of the mainboard.

The plus about pico psu replacement (besides the fact that there is less heat) is that the console will work anywhere in the world, it's only the external power brick that needs to suit mains power. I don't need to use step up converter for my DC now :)

My original Japanese PSU was still working fine, it is now boxed away with the original GD-ROM drive.
 
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therobotx5

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Mar 14, 2021
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I final got my 100 ohm RAs in and here are my results.PXL_20210323_190730974~2.jpgPXL_20210324_225538707~2.jpgPXL_20210323_190655469~2.jpgPXL_20210324_225359576~2.jpg
And it works. GDEMU installed in a VA0.
PXL_20210324_232353979.jpg

Now although it works I have not tested for a long period of time but based on the previous post and the explanation of the mod I would say this is a good solution.

And anyone wondering why the case is warped and dirty, this dreamcast survived a house fire.PXL_20210324_234332230.jpg
 

deadlegion

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even if the console does not restart with the rest of the games, the power supply will still work at its limit and with prolonged use it may fail
Interesting, I only used a 2A for initial testing.
Is a Wii power brick ok to use (3.7A if I recall correctly?).
I'm using 4.1A with my DC at the moment but I had to change the plug to correct type (hence why I used 2A to begin with).
 

FamilyGuy

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Interesting, I only used a 2A for initial testing.
Is a Wii power brick ok to use (3.7A if I recall correctly?).
I'm using 4.1A with my DC at the moment but I had to change the plug to correct type (hence why I used 2A to begin with).
I've been using a GameCube brick for years without issues. I think it's 3.7A as well.

As long as you're over 35W, or something like that it should be fine.
 
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truemaster

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now i see this mod. well i have my doubts. sure is gdemu safe since the g1 is on 3.3v now (or nearly there). but the whole g1of the va0 is made for 5v. so a long term testing is the way to tell. i will not be surprized if the console freeze or reboots and not because of the famous psu contact. i want to be wrong though but the va0 vs va1 in term of voltages is not just only resistores but the ic's as well
 

megavolt85

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i want to be wrong though but the va0 vs va1 in term of voltages is not just only resistores but the ic's as well

microcircuits have diodes that limit the voltage to the supply voltage, but there is also a maximum current strength when exceeding which the diodes burn out, resistors are just needed to limit the current

if you don't know, megadrive also has a 5 volt bus, and all old flash cartridges (and those sold on aliexpress) use an FPGA designed for 3.3 volts, but this does not prevent them from working for decades ;)

you can argue for a long time whether it is right or not, but it has been working for more than one decade and while some argue, others use

P.S. one friend of mine is engaged in the wholesale purchase of retro consoles in japan, he remade 30 consoles according to this scheme and sold them, before that he could not get rid of the VA0 batch because no one wants to play from the drive, and GDEMU cannot be installed
 

truemaster

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aside from what happen in megadrive you cant compare 2 different things. well if this va0 modification isn't gona backfire hell yeah why not?
 

truemaster

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what i meen is the megadrive cartridge is rated at 3.3v but work on 5v because is 5v tolerant. gdemu is 3.3v fpga too but not 5v tolerant. you alredy know this. this va0 modification is gdemu safe because is reduce the curent, else the gdemu would had toasted. my concern is the stability of the g1 at long term run because of this power reducing. again this stability concern of mine it isnt sure that will happen but a long term test is required to be sure
 

Modzvilleusa

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This is amazing information! I've been sitting on some Va0s for quite some time and I'm stoked to breath some fresh life into them. Just got an order in for a ton of those 100ohm resistors, they're on backorder though unfortunately. If anyone has some that they're willing to sell let me know! I'd like to bust these DCs out before the next year :D

Update - I found some of these 100ohm Resistor Packages that are in stock on a site called Arrow. Just got an order in, I've never used this site before before but it seems legit. I'll report back once the order arrives and after the mod is done.

 
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Modzvilleusa

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resistors do not lower voltage
the voltage drops on the protective diodes in the FPGA, but these diodes can burn out from the current flowing through them, so that this does not happen, you need to limit the current, this requires resistors, they will limit the strength of the current flow

on the motherboard there are 6 resistor assemblies of 33 Ohm each (one assembly is 4 resistors), they need to be replaced with 100 Ohm resistor assemblies
RA507, RA509, RA510, RA513, RA514, RA515, RA525
Thanks so much for sharing this information!
I went ahead and made a youtube video showing people how to do this. Hopefully this helps to spread the word.
I buy DC lots from Japan and the time and VA0s seem to be significantly more common in the NTSC-J region.
Being able to safely run a GDemu in Va0s is truly a game changer.
Much love my dude!
 

L10N37

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did this mod tonight
one GDemu won't allow a boot at all. The other takes a good while to show the white Dreamcast swirl screen then takes a few minutes to go into the BIOS only to hang on a screen in Japanese, which I think translates to something about a disc read error. tried 2 supplies as I removed 12v rail from the original. No change. the 'working' GDemu was always giving a red light, I reflowed a bunch of connections on it and it seems to have fixed the red light issue, but no change in anything else. they were both extremely cheap.. one off Ali and one off eBay on big discount sales. putting the GD drive back in does the same thing as the 'working' GDemu! triple checked all resistor connections under a scope!
 

megavolt85

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@ L10N37
1) check the resistance of the resistor assemblies with a multimeter, perhaps you were sold 1000 ohms or 10000 ohms with incorrect marking
2) solder all contacts of the CN503 connector
 

Xrider

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The best think is only replace bios and memory flash by 3.3V chip.
My Dreamcast WORK perfectly with genuine Power Supply, and no bug or other think with this method.
I understand this mod is not easy to do, but this method is simply the best thing.

The resistor replacement is something that works, but seen with an electronic eye is something that looks like bad think...
All information is at the top of this topic ;)


Capture-d-e-cran-2020-08-28-a-09-47-47.png

Dreamcast Service manual is the best reference, and it's say the bus need only 3.3 logic level for VA1 (VS 5V for VA0). Ofcourse, Holy is 5V tolerance and is on VA1 and VA0 ;)

Series resistors in line with 3.3V flash parts (such as the Krikzz Everdrive M2 product) is BAD engineering.
The maximum Vin those parts can handle is 4V, after that, all excess votlage is converted to latch-up current by the input clamping diodes - and also means that your console is sourcing alot of current on the data and address lines.

Conclusion : the resistor array swap work but is not the good think. It's work for how many time before burn your GDEMU 3.3V on 5V logic level on your VA0 ?
If imporant to know : The GDEMU on VAO don't burn immediatly, it's work sometime with no modification, but it will dead after some time...

The resistor array reducing current, but 5V is 5V.
Is not good to compare this method with Evcerdrive MD2 (with cpld MAX2) with resistor array
MAX2 Cpld on Everdrive MD2 cartridge have diode clamp on I/O.
Cylcone too, but you can check on the new Everdrive, krikzz have add logic level transceiver on her new everdrive with FPGA Cyclone and he have delete the series resistors ;)



This is the doc of Altera MAX2 for this :

Capture d’écran 2021-11-11 à 08.47.03.png


Regards,

Xrider
 
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